A 70-year-old man with a past history of lung resection for early stage lung cancer was admitted to our hospital because of worsening exertional dyspnea. Right heart catheterization revealed severe pulmonary arterial hypertension (PAH) with pulmonary vascular resistance of 1671.64 dyne.sec.cm(-5). The patient was treated with sildenafil added to an oral prostacyclin analog, beraprost, and long term oxygen therapy. His exertional dyspnea continued to improve until his sudden death following nasal bleeding. Autopsy revealed marked thickening of pulmonary arteriolar walls, but no recurrence of lung cancer, significant pulmonary embolism or pulmonary parenchymal disease. His PAH could not be explained by the mild airway obstruction or sleep apnea syndrome, and unrelated pulmonary vascular disease was suspected.